A valve positioner system controls the position of a valve in response to a setpoint signal. For example, in a chemical mixing process, a valve positioner system may be used to regulate flow to control a concentration of a particular chemical in the mixing process. A chemical mixing process controller monitors the concentrations of all chemicals in the mix and provides setpoint signals to various valve positioners which control the flow of the chemicals.
A valve positioner system controls position of the valve by comparing a measured position with a setpoint or target position and changing an output used to control the position of the valve accordingly. The valve positioner system typically includes a current-to-pressure (i/p) transducer that receives a variable electrical input signal and provides a pneumatic output signal. The valve positioner employs the i/p transducer in a feedback loop that includes a pneumatic relay, a valve actuator, a valve having a valve stem attached to a valve flow modulating member, a positioner feedback linkage, a position sensor, and a microprocessor.
The microprocessor receives the setpoint signal and produces the input signal for the i/p transducer. The valve actuator responds to the pressure change produced by the pneumatic relay by driving the valve stem to control the degree to which the valve flow modulating member is open. The positioner feedback linkage transmits the position of the valve stem to the position sensor, which provides a signal indicative of the position to the microprocessor. The microprocessor then adjusts the signal supplied to the i/p transducer so as to move the valve flow modulating member to the position indicated by the setpoint, or to maintain the position of the valve flow modulating member at the setpoint.